Si je ne me trompe pas il faut faire un create avant le save:
public function add() {
if ($this->request->is('post')) {
$this->User->create();
if ($this->User->save($this->request->data)) {
$this->Session->setFlash(__('The User has been saved.'));
return $this->redirect(array('action' => 'index'));
} else {
$this->Session->setFlash(__('The license could not be saved. Please, try again.'));
}
}
}